The epidermal growth factor receptor family (EGFR or ErbB/HER family) is a subgroup of the receptor tyrosine kinases (RTKs) and includes four members: EGFR/ErbB, HER2/ErbB2, HER3/ErbB3 and HER4/ErbB4. The members of the EGFR family are closely related single-chain modular glycoproteins with an extracellular ligand binding region, a single transmembrane domain and an intracellular tyrosine kinase domain. In normal physiological settings the ErbB family regulates key events in coordination of cell growth, differentiation and migration. EGFR, HER2 and HER3 are believed to play crucial roles in the malignant transformation of normal cells and in the continued growth of cancer cells. EGFR and HER2 have been found to be overexpressed by many epithelial cancers. Overexpression of EGFR and HER2 has furthermore been linked to disease progression, reduced survival, poor response and chemotherapy resistance in several human epithelial cancers. The role of HERO in malignant transformation and cancer progression is controversial and will not be discussed further here.
EGFR and HER2 are validated cancer targets and both monoclonal antibodies and small molecule inhibitors of their tyrosine kinase have been approved for the treatment of various cancers. HER3 is currently being explored as a potential therapeutic target. However, patients who initially respond to these therapies often relapse due to evolvement of acquired resistance. Pre-clinical research points to the involvement of the one or both of the non-targeted receptors in the development of resistance. Thus, it appears that the ErbB receptors have the ability to replace one another in order to maintain growth stimulatory signaling and a malignant phenotype. Simultaneous targeting of two or all three receptors could therefore be a more efficient way of inhibiting cancer cells with ErbB family dependency.
EGFR is a 170 kDa cell surface glycoprotein consisting of a single polypeptide chain of 1186 amino acid residues as originally determined and described by cloning and sequencing of human cDNAs from a human vulval carcinoma cell line. EGFR contains three major domains: an extracellular domain, a transmembrane domain and an intracellular domain containing the tyrosine kinase. The catalytic activity of EGFR resides in the tyrosine kinase domain (residues 685-953) and is activated upon ligand binding.
The EGFR exists in two different conformations, namely a tethered conformation (closed) and an extended conformation (open). The receptor shifts between the two conformations. In the tethered conformation domains II and IV of the extracellular region of EGFR interact, leaving the receptor in an autoinhibited state. Furthermore, domain III is held at a significant distance from domain I, whereby binding of EGF to both domains simultaneously is impossible. In the extended conformation of EGFR, domains I, II and III are sterically arranged in a C shape, giving room for EGF binding. Furthermore, the conformational changes induce exposure of a β-hairpin consisting of a 20 residue region in domain II, also known as the “dimerization arm”. The dimerization arm extending from domain II of the EGFR makes extensive contacts with the domain II of another EGFR, thereby forming an EGFR homodimer.
Dimerization brings the active cytoplasmic tyrosine kinase domains of the receptors close enough for phosphorylation of the tyrosine residues in the regulatory regions of the receptors. Furthermore, the juxtamembrane regions of the two receptors form an antiparallel dimer which has been found to be important in stabilizing the tyrosine kinase dimer. The “receptor-mediated” dimerization mechanism is unique for the ErbB family compared to other tyrosine kinase receptors where “ligand-mediated” dimerization is the more common theme.
A number of modes of activation of the intracellular tyrosine kinase domain of EGFR have been suggested. Unlike other receptor tyrosine kinases, the EGFR tyrosine kinase domain by default adopts a conformation normally observed only in phosphorylated and activated kinases. This indicates that the kinase domain of EGFR is constructively active. Regulation of a constitutive tyrosine kinase would thus occur through the delivery of a dimerization partner's C-terminal regulator region for trans-phosphorylation. Another possibility is that activation of the tyrosine kinase domain involves displacement of inhibitory interactions that have not been visualized in crystallographic studies. However, crystal structure analyses of the juxtamembrane and tyrosine kinase of EGFR have revealed that an asymmetric dimer of tyrosine kinases formed upon dimerization of two EGFRs is important for regulation of the tyrosine kinase activity. In this asymmetric homodimer one of the tyrosine kinases plays the receiver while the other tyrosine kinase plays the donor. Only the receiver kinase domain has catalytic activity and proceeds to phosphorylate tyrosine residues in the C-terminal tail of the receptor (whether in cis or trans, or both is unknown).
The clathrin-mediated endocytosis is the most important mechanism of down-regulation of EGFR. The destiny of EGFR depends on the stability of the ligand-receptor complex. Upon EGF binding to EGFR the EGFR homodimer is rapidly targeted to clathrin-coated pits and internalized through ligand-induced endocytosis. Simultaneously, EGFR is heavily ubiquitinated by the attachment of both monoubiquitin and polyubiquitin. The ubiquitin ligase Cbl is responsible for the ubiquitination of EGFR. Cbl binds either directly or indirectly through an adaptor protein such as Grb2 to phosphorylated tyrosine residues at the regulatory region of EGFR. The binding of Cbl to EGFR via Grb2 is necessary for receptor internalization. Esp15 also plays a role in EGFR internalization. The exact role of Esp15 is however still controversial. The ubiquitination is involved in endocytotic downregulation of EGFR and endosomal sorting of EGFR to lysosomes. The ubiquitin chains are recognized by the endosomal sorting complex required for transport (ESCRT) and the Hrs/STAM, which retains ubiquinated proteins in the membrane of early endosomes, thereby hindering recycling of EGFR. Subsequently, EGFR is sorted into intra luminal vesicles (ILVs), which leads to delivery of EGFR to the late endosome and finally degradation in the lysosomes.
In contrast to the degradation of EGFR when bound to EGF, TGF-α binding allows receptor recycling. The TGF-α ligand dissociates rapidly from EGFR in the early endosome due to the acidic environment, leading to receptor dephosphorylation, de-ubiquitination and thereby recycling of the receptor back to the cell surface.
Human epidermal growth factor receptor 2 (HER2, ErbB2 or Neu) was first described in 1984 by Schechter et al, HER2 consists of 1234 amino acids and is structurally similar to EGFR with an extracellular domain consisting of four subdomains I-IV, a transmembrane domain, a juxtamembrane domain, an intracellular cytoplasmic tyrosine kinase and a regulatory C-terminal domain.
The domain II-IV contact that restricts the domain arrangement in the tethered EGFR is absent in HER2. Three of the seven conserved residues important for stabilizing the tether in the unactivated EGFR are different in HER2. HER2 thus resembles EGFR in its extended (open) form with the dimerization arm exposed and apparently poised to drive receptor-receptor interactions. The absence of a tethered HER2 conformation indicates that the receptor lacks autoinhibition as seen for the other members of the ErbB family. A stable interface of subdomain I-III seems to keep HER2 in the extended configuration similar to the extended configuration of the EGFR-EGF complex. The interaction between domains I and III involves regions corresponding to ligand-binding sites in domains I and III of EGFR, leaving no space sterically for ligands, rendering HER2 incapable of binding ligands. Domains II and IV form two distinct interfaces that stabilize the heterodimer formation of HER2 and another member of the ErbB family.
Biophysical studies have failed to detect significant HER2 homodimerization in solution or in crystals. The residues of domain II of EGFR and HER2 are similar. However, Arg285 at the dimer interface is not conserved between EGFR and HER2. In HER2 residue 285 is Leu. Mutation studies indicate that Leu at this position is partly responsible for the absence of HER2 homodimers in solution. Dimerization of intact HER2 in vivo may require additional interactions of sites in the transmembrane domain of HER2.
HER2 is the only member of the ErbB family that does not bind known ligands, HER2 is instead activated via formation of heteromeric complexes with other ErbB family members and thereby indirectly regulated by EFGR and HER3 ligands. HER2 is the preferred heterodimerization partner of the three other ErbB receptors. HER2 enhances the affinity of the other ErbB receptors for their ligands by slowing down the rate of ligand receptor complex dissociation, whereby HER2 enhances and prolongs signaling. The ability of HER2 to enhance the ligand affinity of other ErbB receptors may reflect the promiscuous behavior of HER2 as a heterodimerization partner. Heterodimerization of HER2 and another ligand-hound receptor of the ErbB family induces cross-phosphorylation, leading to phosphorylation of the C-terminal tyrosine residues. The most active HER2 heterodimer is the HER2-HER3 complex, HER2 complements the kinase-deficient HER3 by providing an active kinase.
In contrast to EGFR, HER2 is internalization resistant when overexpressed. Overexpression of HER2 has further been reported to inhibit endocytosis of the other ErbB family members. Two mechanisms by which HER2 escapes lysosomal degradation and thereby remains at the plasma membrane have been suggested. Either HER2 avoids internalization or it becomes efficiently recycled from endosomes back to the plasma membrane. Studies using labeled antibodies have shown that HER2 is constantly internalized and recycled. Other studies in contrast failed to identify intracellular HER2 in cells treated with compounds known to inhibit recycling.
It has been proposed that the carboxyl terminus of HER2 does not possess all signals required for internalization or that it contains an inhibitory signal essential for clathrin-mediated endocytosis. Additionally, studies have shown that HER2 heterodimers are not delivered to endosomes. A Cbl docking site like the one found on EGFR has also been identified on HER2 (Y1112). Cbl can thereby be recruited to HER2, leading to ubiquitination of HER2, but the actual binding efficiency of Cbl is unclear. It has been proposed that HER2 is internalization resistant due to its association with membrane protrusions. Finally, other studies have shown that the endocytosis resistance of HER2-EGFR heterodimers is associated with inefficient EGF-induced formation of clathrin-coated pits.
The third member of the ErbB family, known as human epidermal growth factor receptor 3 (HERB, ErbB3) was identified in 1989 by Kraus M. H. et al. The HER3 gene encodes a protein of 1342 amino acids with striking structural similarities to EGFR and HER2. Features such as overall size, four extracellular subdomains (I-IV) with two cysteine clusters (domains II and IV), and a tyrosine kinase domain show structural similarities to EGFR and HER2. The tyrosine kinase domain of HER3 shows 59% sequence homology to the tyrosine kinase domain of EGFR.
Just like EGFR, HER3 exists in a tethered conformation and an extended conformation. In the tethered conformation the dimerization arm is buried by interactions with domain IV, leaving domains I and III too far apart for efficient ligand binding. Ligand binding to the extracellular domains I and III occurs in the extended conformation of HER3 and leads to heterodimerization with other members of the ErbB family. No HER3 homodimers are formed upon ligand binding. The extended and ligand-bound HER3 molecule preferentially heterodimerizes with HER2.
In contrast to EGFR and HER2, the tyrosine kinase of HER3 has impaired catalytic activity, insufficient for any detectable biological response. Two amino acid residues which are highly conserved in the catalytic domains of protein kinases are altered in the catalytic domain of HER3. These are the substitution of aspargine for aspartic acid at residue 815 and substitution of histidine for glutamate at residue 740. The two amino acid substitutions may be the reason why HERB lacks catalytic activity of its tyrosine kinase domain. Because of the impaired intrinsic kinase activity of HER3 the receptor needs to heterodimerize with another ErbB family member in order to respond to its own ligand binding.
Little is known about endocytosis of HER3. Moreover, different studies have suggested that HER3 is endocytosis impaired to the same extent as HER2. In agreement with this, the HER3-NRG1 complex was found to be internalized less efficiently and slower than the EGFR-EGF complex, supporting the view that HER3 is not endocytosed as efficiently as EGFR. However, when the C-terminal tail of EGFR was replaced with the C-terminal tail of HER3, EGFR became endocytosis impaired, suggesting that a region in the C-terminus of HER3 protects the receptor against internalization. It has also been suggested that NRG1 does not efficiently target HER3 to degradation due to the dissociation of the ligand-receptor complexes in endosomes, as it is observed when EGF is activated by TGFα.
Targeting the ErbB family has been intensely pursued in the last decade as a cancer treatment strategy. Different treatment modalities have been explored, such as tyrosine kinase inhibitors (TKIs), monoclonal antibodies (mAbs) and ligand-traps. An advantage of monoclonal antibodies for treatment of cancer is target specificity, ensuring a low toxicity compared to conventional cytotoxic cancer chemotherapy. Monoclonal antibodies have been approved for the treatment of solid tumors with abnormally high levels of EGFR or HER2, and numerous mAbs targeting EGFR or HER2 are in clinical trials. TKIs inhibit receptor signaling by binding to the ATP-binding site in the tyrosine kinase domain of EGFR and HER2. Erlotinib/Tarceva® inhibits tyrosine kinases of EGFR while lapatinib/Tykerb® inhibits tyrosine kinases of both EGFR and HER2. Both erlotinib and laptinib are FDA approved TKIs for use in the treatment of non-small lung cancer (NSCLC) and HER2 overexpressing metastatic breast cancer, respectively.
However, despite the clinical usefulness of monoclonal antibody therapy and TKIs, development of acquired resistance to the treatment is an increasing issue. Combination therapy of mAbs and conventional cytotoxic chemotherapy is one of the approaches being carried out in order to increase treatment efficacy. Furthermore, several strategies are being explored to increase the efficacy of monoclonal antibodies, including enhancement of effector functions, and direct and indirect arming of the antibodies with radionuclides or toxins.
Thus, a need exists for additional drugs to treat EGFR family-related diseases in patients, including patients who have developed resistance to existing treatments. These additional drugs also should have a low risk of provoking an undesirable immune response when used to treat human patients.